Can we create a VM using Azure Resource Manager in a Virtual Network that was created using classic deployment?

It is not supported by the Azure portal. We cannot use Azure Resource Manager to deploy a Virtual machine into a virtual network which was created using classic deployment.

No, it is not possible to create a VM using Azure Resource Manager (ARM) in a Virtual Network that was created using classic deployment. Azure Resource Manager and classic deployment models are not compatible with each other. Virtual Networks created using the classic deployment model cannot be directly used in ARM deployments.

If you want to create a VM using ARM, you should create a Virtual Network using the ARM deployment model. It’s recommended to use the ARM deployment model for new deployments as it provides more features, flexibility, and is the preferred model for Azure resource management. If you have existing resources deployed using the classic deployment model, consider migrating them to ARM for better management capabilities and compatibility with newer Azure features.